It disturbs me when I hear Christians haphazardly say ALL people are "children of God" no matter who they are or what they believe.

Scripture clearly teaches that only those who believe on the Lord Jesus Christ receive that positional title which defines our relationship to Him and sets us apart from unbelievers.

John 1:12
But as many as received Him, to them He gave the right to become children of God, to those who believe in His name...

Romans 8:15-17
For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, “Abba, Father.”

The Spirit Himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together.